Apparatus for pre-stressing concrete structures



June 21, 1955 R. J. KENNEDY APPARATUS FOR PRE-STRESSING CONCRETESTRUCTURES 2 sheets-sheet 1 Filed Aug. 14, 1953 June 21, 1955 R. J.KENNEDY 2,711,291 i APPARATUS FOR PRE-STRESSING CONCRETE STRUCTURESFiled Aug. 14, 1953 2 Sheets-Sheet 2 United States atent APPARATUS FORPRE-STRESSIN G CONCRETE STRUCTURES This'invention concerns method andmeans for applying pre-stressed banding material, e. g. steel wire, toan object, e. g. a concrete tank or pipe.

The advantages claimed over previous methods and means are: (1) Theapparatus is small and compact enough to be used without disturbingscaffolding used in construction of tanks; previouslyscaifolding has hadto be removed and replaced. (2) Its power consumption is so small as tomake operation feasible with portable electric power plant. (3) Accuratecontrol of tension in wire permits closer, more economical design ofstructure to be reinforced. (4) A graph showing actual tensioneverywhere in wire can be obtained automatically, thereby assuring thatthe work meets specifications.

Reference is made to the accompanying drawings wherein:

Figs. 1 and 2 show diametral sections of two different forms of the unitwhose rolling movement along the object placesand stresses thereinforcing material.

Fig. 3 shows such a unit mounted on a carriage in position on a tank orthe like, hereafter called the structure.

Fig. 4 is a simplified plan view of the unit mounted as in Fig. 3 and asseen from just below the supporting member. 2 i

In Fig. 1 is shown a motor 1 which by way of shaft 2 through the fixedmember, 3 and the reducing train of gears 4, 5, 6 and 7, produces atorque tending to rotate the coaxially mounted members 8 and 9 withrespect to each other and the fixed member 3. The member 8 has teetharound its-periphery-which are intended to engage a continuous chain tobe reinforced and is kept at sufficient tension to prevent seriousslippage. The peripheryof member 9 is a'surface of revolution ontowhichthe banding wire 10 is guided beneath the roller 11 and from asmaller radius of which the wire unrolls onto the object.

the wire, in which in the operation of the device the tension steadilyincreases as it progresses toward the point of leaving 9, shall slideand roll smoothly toward that point. The roller 11 is pressed(preferably by a spring) toward the axis sufiiciently to preventslippage of wire on 9 at point of rolling-on, but can be forced away toallow passage of a compact coupling on wire. The meter 12 is intendedschematically as an indicator or recorder of the torque of motor andtherefore of tension in the wire.

The motor 1 is intended to exert a constant torque regardless of itsspeed; this result can proceed from design of motor or from an auxiliaryarrangement, e. g. a constant current regulator with a direct-currentmotor. Such regulation is simple because the power required is small.Since the final tension in the wire is proportional to this torque, thetension will then be independent of the speed of operation of themachine and of slippage of the chain along which it travels. 7

It can be shown readily that if the final tension in wire is T, theradius of 9 at which wire is unrolled is Rs, torque or the like whichencircles the object Q The shape of the sure face" of '9 is so designedas to insure that every element of I Patented June 21,1955

of motor is L and radius of pitch-circle of 8 is Ra, then the unit isurged forward by a force F R was I t The algebraic sign of the smallvterm involving 'L depends on number of is over-simplified in thedrawing. In this embodiment I prefer a value of a is such that force Fis nearly equal to the frictional force involved in the forwardmovementof the unit and its immediate carriage, i. e. that carried bythe frame-work in Fig. 3, which will be discussed later. Since thefrictional forces are roughly proportional to T, such a value of R8 willensure that only a small force need be exerted by the frame-Work on thetensing unit for any value of T; this fact becomes particularlyimportantwhere high structures are concerned.

In this embodiment the ratio of the radius at which the wire is rolledonto member 9 to the radius R9 aflfects the speed-of rotation of'9, butwire; I V v p In Fig. 2, the member 21 is a motor (presumably withgear-reduction) coupled to the member'22 whichexcept for lack ofinternal gear-teeth is similar in structure and not the final tension ofthe function to the member 8 of Fig. 1. ,Coaxialwith 22 is the member 23which likewise corresponds to member 9 of Fig. 1. One end of a strongspiral spring 24 is attached to 22 and the other end to 23. Near .theirperipheries, 22 and 23 are provided with meshing screw threads 25 sothat their distance apart along axis depends on their relative angularorientation which can be read on a stroboscopically illuminated scaleabout axis. The banding wire 26 shown wrapped about 23 rolls on underroller 27 at a radius of 23 which number of turnsof wire on 23 and thedistance apart of 22 and 23; the roller is so supported as to be capableof appropriate small movement in response to relative movement of 22 and23 andv to be adjustable in orientation about axis or" 22 bodiment.

The action of this device in automatically maintaining.

and since 1' is necessarily less than 1 and S can not be negative, Rmustbe less than R Suppo'se now that for some reason the tension falls belowthe desired value; then spring 24 relaxes and rotates member 23 withrespect to member 22 in such direction as to decrease their separationby the action of screw25; wire 26 is thus allowed to slide into regionof periphery of 23 where radius of rolling-on is decreased; member 23therefore rotates at slightly greater speed than 22, so that tension inspring 24, and consequently that in wire, increases until initial valueis restored, when, too, a new value of R will have been established, ifrequired. The action in case of gears in the reducing train, which isdetermined by the and 23, but always presses these'veral" turns of wirefirmly together at right angles to theirlength, and tightly enoughagainst 23 'to prevent'slipping, except a in the case mentioned at endof discussion of this emanaccidental increase in tension is evidentlyjust the opposite, but again restores initial value of tension. It willbe" evident that'for' accurate regulation it is"desi'rable to have asmall rate of changein tension of spring with angular displacementbetween 22 and 23;.hence it might be desirable 'to use a fusee or longtorsion spring; asmall, constant-torque motor, geared as shown in Fig 1,would act as spring with rate of change zero; the spiral spring has beenshown here merely for. simplicity of representa tion- Instead of relyingfor regulation on the above-discussed vari tion .inlradius, evidentlythe same screw-action could be made to vary the force with which roller27 presses the wire into contact with member 23, and so let wire slip todegree required for regulation.

In Fig. 3 a unit 31 such'as those of Figs. 1 and 2 is shown-mountedcan-a horizontal beam 32 which can move vertieall'y in the frame-work33. The unit is held out of contact 'with surface by rollers34 as itmoves along chain 35 aand;"pays. -outthe wire- 36. Frame-work 33 issuportedJbyZcarIiage 37 which is propelled along parapet 38 of structureto be banded on rollers 39 driven by motor 40. The wire is led by way ofguides {not shown) to unit 31 from coil or spool il'mounted on aturn-table 42.

The unit 31 can be moved manually or automatically invertical directionso "that the wire will be applied spirally as the machine"travels'around the structure. It willbe evident that thegeneral-arrangement enables the unit to be brought to very bottom ofstructure, where usually a great many turns of the banding must beapplied.

"Because the motor attached to unit 31 "supplies, just enough'force' topropel the unit, the frame-work 33 can be lightly built.

*The planview Fig. 4shoWs unit 31 above referred to, and rollers 51which hold chain 35 in positive engagement with sprocket of 31' andare'so oriented about axis of 31 as to supply tension necessary to keepchain from slipping along structure. Means for supporting rollersadjustably are perhaps 'too obvious to require representation. Therollers' 34 above referred to are above plane of view and so not shownas holding'31 away from structure 52.

The statement that the power required in these devices is small,which'ha's'been emphasized in the foregoing discussion, is confirmed inthe following computation of power under conditions that have occurredin practice. Iris easily shown that if friction of moving parts isdisregarded, the'power'required (in horse-power units) is T' s :ZXSSOMAwhere Tstcnsioninponnds s= lineal speed. of machine in' ft.'per sec.M=Youngs modulus for material of wire A=cross-sectional area of wire insq. in. For "T -2500, s=7.5, M =2.8 l' and diameter of wire=0.15, theformula gives Power=0.10 H. P.

I claim:

1. In apparatus for reinforcing a concrete structure by banding itsconvex'su'rface with prest'ressed wire, means comprising a memberarranged to roll tractively along the surface, a second member coaxiallyrotatable with respect to the first and having a region which is acoaxial surface of revolution onto which the Wire is to wind and fromwhich to unwind onto the structure, a roller resiliently holding thewire against the second member at the point of application of the wireto the second member to prevent slipping at that point between the wireand the second member, a motor; geared to both'rotating members so as toproduce a torque resulting in tension in the wire as it unrolls, 'a'mounting for the-above mechanism, a-framework supporting the mounting atvarious altitudes from top to bottom of "the structure and itselfarranged to roll along the top of the structure, a second motor on theframework for driving it circumferentially about the structure, means onthe framework for varying the elevation of the mounted unit as thebanding proceeds, and a source of wire on the framework.

2. Inapparatus. set forth in claim l-an arrangement of gearing of thefirst-mentioned motor, and avalue of the ratio between the radiusat'which the wire leaves the surface ofwrevolution of the second memberand the radius at which the first member rolls along thestructure suchasv to produce a force on the suspended unit in the forward directionapproximately equal to the'overall frictional force opposing the motionof that unit.

3. In combination with the apparatus set forth in claim l,.-a meterindicating the tension in the wire by means of the electriccurrentsupplied to'the motorproviding the stressing effort.

4. In apparatus for reinforcing a concrete structure by banding itsconvex surface with prestressed wire, means comprising a member arrangedto roll tractively along the surface, a secondmember coaxially rotatableto a limited extent with respect to the first and having a region whichis a coaxial surface of revolution of varying radius onto which the wireis to wind and from which to unwindunder tension ata smaller radius,engaging screw-threads in the first and second members whereby a changein their relative angular. displacement. results in a compensatingchange. in radius of rolling-on ofthe wire onto the second member, -a.roller resiliently holding the wire against the second member-at thepoint of application to prevent slipping of. the wireat that point, anda spring joining the two membersandopposing their relative-angulardisplacement.

References Cited. in' the-file. of this .patent UNITED rSTATES :PATENTSHirsh Aug. 29, 1950 V

